Currently, one of the most ubiquitous cellular communication system is a 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving cell during initial access or handovers. Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD). In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and in the same time intervals. In TDD user separation is achieved by assigning different time slots to different uses in a similar way to TDMA. However, in contrast to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions.
An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In these communication systems, it is known to include known data sequences in the transmissions over the air interface in order to allow the receivers to determine characteristics of the received signal and in particular to determine channel estimates for the communication channel between the transmitter and the receiver.
In communication systems such as UMTS the training data is selected by the transmitter in a predetermined fashion allowing the receiver to know which training data is transmitted. Specifically, the Technical Specifications of UMTS define a number of training sequences in the form of midambles that can be used by a transmitter. The receiver may in accordance with the specifications of UMTS not know exactly which training data has been used but will know that a midamble has been selected from a specific set of predefined midambles.
In this case, the receiver may evaluate all possible midambles and determine the transmitted ones in accordance with a suitable criterion.
One particularly efficient method of generating multiple midambles is used in the UTRA TDD mode defined by the 3rd Generation Partnership Project (3GPP). In this technique a single periodic base code is used to derive multiple midambles by cyclic shifts of the periodic base code. Channel estimation may be achieved efficiently at the receiver by performing a single cyclic correlation on the training portion of the burst and segmenting the result. In UTRA TDD, each of these midambles (derived from a single base code) is associated with one or more spreading codes. Thus, in UTRA TDD each spreading coding is linked to a specific midamble.
UTRA TDD currently use a single base code per cell with one or more midambles (shifts of the base code) being used in a single timeslot. One or more spreading sequences is associated with each of these midambles such that different spreading codes may have the same midamble whereas different midambles are linked to different sets of spreading sequences. In some cases, the same midamble may be used for all transmissions from a given transmitter and in particular the same midamble may be used for all transmissions from a given base station.
In order to further improve the performance of cellular communication systems, new techniques are continuously researched, developed and introduced to the standardisation committees.
One such technique is the Multiple-Input-Multiple-Output (MIMO) transmission technique. In this technique, communication is based on a plurality of transmit and receive antennas. Specifically, rather than merely providing diversity from spatially separated transmit antennas, MIMO techniques utilise transmitters having at least partially separate transmit circuitry for each antenna thus allowing different sub-signals to be transmitted from each of the antennas. The receivers may receive signals from a plurality of receive antennas and may perform a joint detection taking into account the number and individual characteristics of the plurality of transmit antennas and receive antennas. This may substantially improve the spectral efficiency of the cellular communication system.
However, current UTRA TDD systems, as defined in Release 5, 6 or previous versions of the 3GPP Technical Specifications, were not designed to support MIMO transmissions and therefore the benefits of MIMO techniques cannot be fully exploited. Thus, a number of disadvantages results from the restrictions of the current Technical Specifications.
In particular, the receivers ability to determine the source of a transmit signal or the channel estimate for a particular antenna is hindered by the requirements and specifications for selecting midambles. For example, in accordance with the current 3GPP Technical Specifications, each burst transmitted from a cell may be assigned a distinct spreading code which is associated with a specific midamble.
Accordingly, in MEMO systems where multiple bursts may be simultaneously transmitted from different transmit antennas using the same spreading codes, the midambles codes from the different transmissions will also be the same resulting in reduced channel estimation and source identification at the receiver. In particular, it may prevent or impede identification of a specific antenna source of the plurality of transmit antennas. Also the maximum number of different midamble sequences that may be used by a cell is limited to the maximum number of spreading codes allowed thereby significantly restricting the capacity of the system
Hence, an improved MIMO system would be advantageous and in particular a system allowing for increased flexibility, improved performance, and/or improved training sequence selection would be advantageous.